The present invention relates generally to a numerically controlled machine tool, such as grinder. More particularly, the invention relates to such a machine tool having a function of correcting a machining dimension of a workpiece and/or a function of setting a tool length after truing or dressing has been performed.
Conventionally, a machine tool equipped with a numerical control (referred to "NC") apparatus has been so designed as to correct machining dimension of a workpiece by changing or setting a tool correction value. In the case where the correction value is determined in accordance with only tool dimension measured by a tool presetter, such conventional machine tool cannot correct other factors such as deformation or displacement of components and tools thereof owing to thermal stress or abrasion of tools. Therefore, when such conventional machine tool is used for high precision machining, it is necessary to correct the tool correction value by actually measuring a dimension of the workpiece after being worked by the machine tool.
In the conventional NC type machine tool, the workpiece should be dimensioned from the machine tool to measure the dimension of the workpiece.
Another conventional machine tool equipped with an automatic tool changer (ATC) such as machining center has been provided with a measuring device assembled on a tool holder as one of the tools. In such machine tool, the dimension of the workpiece is measured by the measuring device which is mounted on a main spindle in the same manner as oridinary tool exchanger.
The former type machine tool, however, may cause a problem that a relatively long working time is required to measure the workpiece after dismounting it from the machine tool and a skilled operator is also required to precisely correct the tool correction value in accordance with the measured data.
The latter type machine tool with ATC must temporarily replace its working tool with the measuring tool to measure the dimension of the workpiece, thereby resulting in time-loss. Although the working tool is remounted on the machine tool after measurement, mounting state such as mounting angle and alignment of the working tool with the main spindle cannot be completely identified with the former mounting state before measurement. This may cause another problem that the measured data is not directly reflected in the tool correction value.
On the other hand, the conventional machine tool equipped with an NC device has been operated according to a processing program which is arranged with use of a correction of tool length in order to adapt the machine tool for general purpose. In a grinding machine, it is necessary to perform truing or dressing for its grinding tool in order to restore the tool to its original shape and sharpness. After truing or dressing, the tool length is measured and then the tool correction value is corrected.
In prior arts, the grinding tool should be dismounted from a main spindle to measure the tool length by a tool presetter and then to correct the tool correction value in response to the measured tool length.
On the other way, the grinding tool mounted on the main spindle, without dismounting from the machine tool, has been brought into contact with a reference block of which dimension is previously measured to measure the tool dimension and the measurement value is set into the program. Further, the grinding tool has been brought into contact with a touch switch stationarily arranged in stead of the reference block to automatically measure the tool dimension.
The first type grinding machine, however, may cause a problem that a relatively long working time is required to measure the grinding tool after dismounting it from the main spindle. Further mounting state such as mounting angle and alignment of the grinding tool on the main spindle can not be completely identified with the mounting state before measurement. This may cause another problem that the measured data is not directly reflected to the correction value.
The second type grinding machine with the reference block should be carried out with manual operation to measure the tool length, thereby resulting in time-loss and demanding a skilled operator.
Further, the third type grinding machine with the touch sensor stationarily mounted on the machine cannot measure the changed (reduced) amount of the grinder for truing or dressing which is ground with the grinding tool.
The above three conventional machines cause a common problem that the program data for truing or dressing has to be corrected whenever the truing or the dressing is carried out, therefore time-loss is caused.